Most modern vehicles have power steering in which the force exerted by the operator on the steering wheel is assisted by hydraulic pressure from an electric or engine-driven pump. The rotary motion of the steering wheel is converted to linear motion in a steering gear. The force applied to the steering wheel is multiplied by the mechanical advantage of the steering gear. In many vehicles, the steering gear is of a rack and pinion type, while in others it is recirculating ball type.
Electric power steering is commonly used in the hybrid vehicles to improve fuel economy. This is accomplished through the reduction of losses inherent in traditional steering systems. When operating at low speeds, hydraulic assist provides satisfactory feel and response characteristics and accommodates the excess capacity required for high-speed operation by the constant circulation of hydraulic fluid through a directional bypass valve. This bypass flow combined with system backpressure expends power needlessly from the vehicle powerplant. These losses are also a function of the rotational speed of the pump. Thus, hydraulic efficiency decreases with engine speed. Average losses under a no steering, zero speed condition can exceed 100 Watts.
Alternatively, electric power steering requires power only on demand. The electronic controller requires much less power under a no steering input condition. This dramatic decrease from conventional steering assist is the basis of the fuel economy savings. Electric power steering has several additional advantages. The steering feel provided to the operator has greater flexibility and adaptability. Overall system mass savings may also be achieved. Electric power steering is powerplant independent, which means it can operate during an all electric mode on a vehicle.
Furthermore, it is known in the art relating to electric motors that polyphase permanent magnet (PM) brushless motors excited with a sinusoidal field provide lower torque ripple, noise, and vibration when compared with those excited with a trapezoidal field. Theoretically, if a motor controller produces polyphase sinusoidal currents with the same frequency and phase as that of the sinusoidal back electromotive force (EMF), the torque output of the motor will be a constant, and zero torque ripple will be achieved. However, due to practical limitations of motor design and controller implementation, there are always deviations from pure sinusoidal back EMF and current waveforms. Such deviations usually result in parasitic torque ripple components at various frequencies and magnitudes. Various methods of torque control can influence the magnitude and characteristics of this torque ripple.
One method of torque control for a permanent magnet motor with a sinusoidal, or trapezoidal back EMF is accomplished by controlling the motor phase currents so that the current vector is phase aligned with the back EMF. This control method is known as current mode control. In this a method, the motor torque is proportional to the magnitude of the current. However, current mode control has some drawbacks, in that it typically requires a complex controller for digital implementation and processing. The controller also requires multiple A/D channels to digitize the feedback from current sensors, some of which need to be placed on at least some phases for phase current measurements. Another drawback of current mode control is its sensitivity to current measurement errors, which cause torque ripple to be induced at the fundamental frequency.
Another method of torque control is termed voltage mode control. In voltage mode control, the motor phase voltages are controlled in such a manner as to maintain the motor flux sinusoidal and voltage rather than current feedback is employed. Voltage mode control also typically provides for increased precision in control of the motor, while minimizing torque ripple. One application for an electric machine using voltage mode control is the electric power steering system (EPS) because of its fuel economy and ease-of-control advantages compared with the traditional hydraulic power steering. However, commercialization of EPS systems has been limited due to cost and performance challenges. Among the most challenging technical issues are a pulsation at the steering wheel and the audible noise associated with voltage mode control.
Despite the above-mentioned methods, a more precise or accurate control scheme is needed for some applications. For example, in an electric power steering control system, some unidentified faults or failure of a detection function may cause an undesired condition in the system. Generally speaking, current mode control method involves a trapezoidal system employing feedback, whereas the voltage mode control method involves a sinusoidal system wherein no feedback occurs. That is, current mode control is a closed loop methodology in that a current feedback is used as controlling parameter by the current mode control, while voltage mode control is an open loop control methodology. Therefore, in case of an unidentified fault within the voltage control system, or a change in the motor parameters, steering assist might be undesirably affected. Thus, there is a need in the art for a diagnostic method that enables various control methodologies to be employed while reducing the effect of unidentified faults.